This invention generally relates to a process for thermally stress-relieving a metallic conduit that is surrounded in part by a heat sink, such as a section of an Inconel.RTM. heat exchanger tube that is surrounded by a support plate in a nuclear steam generator.
Processes for thermally relieving the tensile stresses which may occur in heat exchanger tubes are known in the prior art. Such tensile stress may occur as a result of a manufacturing or maintenance operation. For example, stress causing bends are incorporated into the heat exchanger tubes used in nuclear steam generators during their manufacture in order to give them their distinctive U-shape. Stress-causing expansions are routinely induced in the sections of these heat exchanger tubes that extend through the tubesheet and support plates of the steam generator, both during the manufacture and maintenance of the generator. Finally, stress-causing welds may be placed around the interior walls of these tubes whenever reinforcing sleeves are welded therein. Other such tensile stresses may occur from the accumulation of sludge deposits in the crevice regions of the generator. The applicants have found that one of the most troublesome sources of such stresses is the sludge that accumulates in the annular region between the heat exchanger tubes and the bores in the support plates through which they extend in the steam generator. Such deposits can accumulate in these annular regions and expand to such an extent that the tube becomes dented into an ovular cross section in the support plate region.
Unfortunately, any tensile stress that is induced into the wall of such a heat exchanger tube may lead to an undesirable phenomenon known as "stress corrosion cracking" if these stresses are not relieved. However, in order to fully understand the dangers associated with such stress corrosion cracking, and the utility of the invention in preventing such cracking, some general background as to the structure, operation and maintenance of nuclear steam generators is necessary.
Nuclear steam generators are comprised of three principal parts, including a secondary side, a tubesheet, and a primary side which circulates water heated from a nuclear reactor. The secondary side of the generator includes a plurality of U-shaped heat exchanger tubes, as well as an inlet for admitting a flow of water. The inlet and outlet ends of the U-shaped tubes within the secondary side of the generator are mounted in the tubesheet that hydraulically separates the primary side of the generator from the secondary side. The primary side in turn includes a divider sheet which hydraulically isolates the inlet ends of the U-shaped tubes from the outlet ends. Hot, radioactive water flowing from the nuclear reactor is admitted into the section of the primary side containing all of the inlet ends of the U-shaped tubes. This hot, radioactive water flows through these inlets, up to the tubesheet, and circulates around the U-shaped tubes which extend within the secondary side of the generator. This water from the reactor transfers its heat through the walls of the U-shaped tubes to the nonradioactive feedwater flowing through the secondary side of the generator, thereby converting feedwater to nonradioactive steam that in turn powers the turbines of an electric generator. After the water from the reactor circulates through the U-shaped tubes, it flows back through the tubesheet, through the outlets of the U-shaped tubes, and into the outlet section of the primary side, where it is recirculated back to the nuclear reactor.
The walls of the heat exchanger tubes of such nuclear steam generators can suffer from a number of different forms of corrosion degradation, one of the most common of which is intragranular cross corrosion cracking. Empirical studies have shown that the heat exchanger tubes may be more susceptible to stress corrosion cracking whenever they acquire significant amounts of residual tensile stresses, whether deliberately induced in the manufacture or maintenance of the tube, or incidentally induced by accumulation of sludge in the crevice regions of the steam generator. If such stress corrosion cracking is not prevented, the resulting cracks in the tubes can cause the heat exchanger tubes to leak radioactive water from the primary side into the secondary side of the generator, thereby radioactively contaminating the steam produced by the steam generator.
In order to prevent such corrosion and tube cracking from occurring in the heat exchanger tubes of the generator, both mechanical and thermal stressrelieving processes have been developed. One of the most successful thermal stress-relieving processes in existence is that which is disclosed in U.S. patent application Ser. No. 24,941 filed Mar. 12, 1987 and entitled "Process for Thermally Stress Relieving a Tube" by Bruce Bevilacqua et al., assigned to the Westinghouse Electric Corporation, the entire specification of which is incorporated herein by reference. This particular process provides results in an extremely fast yet reliable process for stress relieving Inconel.RTM. heat exchanger tubes which have had tensile stresses induced therein by bending, denting, tube expansions, or sleeve weldings.
Unfortunately, this particular prior art process is very difficult to implement along tube sections having nonhomogeneous thermal conductivity characteristics, such as the sections of the heat exchanger tubes that are surrounded by support plates. The sections of the tubes that extend through the support plates often need to be stress relieved either as a result of a deliberate tube expansion in this area, or a radial tube denting in this region caused by the accumulation of sludges in the region between the outside of the tube and the bore in the support plate which expand over time. For a detailed discussion of such radial tube denting and the maintenance expansions designed to prevent them, reference is made to U.S. Pat. No. 4,649,492 by S. Sinha et al., and assigned to the Westinghouse Electric Corporation, the entire specification of which is incorporated herein by reference. However, if one attempts to thermally stress-relieve the section of the tube extending through and in contact with such support plates in the fashion taught by the previously referred to Ser. No. 24,941 application (i.e., by means of a heater assembly that is at least as long as the tube section to be heat treated), one of two unsatisfactory results follow. Either the portion of the tube directly in contact with the support plate will be underheated due to the heat sink properties of the plate, or (if the power of the heater assembly is increased to adequately heat the plate contacting portion of the tube) the sections of the tube above and below the plate will become overheated (i.e., heated to over 1500.degree. F.). Such overheating may cause carbides to precipitate in the grain boundaries of the Inconel.RTM. that forms the tube, thereby rendering these portions of the tube more susceptible to stress-corrosion cracking, and defeating the purpose of the thermal stress relief.
Clearly, what is needed is an improved method for thermally stress-relieving a tube which is capable of creating and maintaining a substantially uniform heat gradient across a section of a metallic tube that is characterized by nonhomogeneous thermal-loss properties. Ideally, such an improved process should be simple to implement by means of a commercially available heater assembly. Finally, it would be desirable if such a process was just as reliable and fast as previously developed thermal stress-relieving processes that are usable on sections of heat exchanger tubes having substantially homogeneous thermal loss properties.